Thin film magnetic recording media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the magnetic recording layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains (bits) of the grains in the magnetic recording layer. FIG. 1, obtained from Magnetic Disk Drive Technology by Kanu G. Ashar, 322 (1997), shows magnetic bits and transitions in longitudinal and perpendicular recording.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hcr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media. For example, as the SMNR is reduced by decreasing the grain size or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases.
Conventionally used storage media contain a magnetic recording layer having Co—Cr—Pt—B and Co—Cr—Ta alloys where B and Ta are mainly used to improve the segregation of Cr in the magnetic layer. A better segregation profile of Cr leads to a sharper transition between the magnetic grains and the non-magnetic Cr-rich grain boundaries, and thus, the recording media is expected to have higher saturation magnetization, Ms and magnetocrystalline anisotropy and narrower intrinsic switching field distribution.
As the storage density of magnetic recording disks has increased, the product of Mr and the magnetic layer thickness t has decreased and Hcr of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hcr. To achieve a reduction in Mrt, the thickness t of the magnetic layer has been reduced, but only to a limit because the magnetization in the layer becomes susceptible to thermal instability. This instability has been attributed to thermal activation of small magnetic grains (the super-paramagnetic effect). Such thermal instability can cause undesirable decay of the output signal of the magnetic recording medium and data loss.
The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic layer and V is the volume of the magnetic grain. As the magnetic layer thickness is decreased, V decreases. Thus, if the magnetic layer thickness is too thin, the stored magnetic information might no longer be stable at normal disk drive operating conditions.
One proposed solution to the problem of thermal instability is to increase Ku. However, the increase in Ku is limited to the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too large to be written by a conventional recording head. On the other hand, a reduction in Mr of the magnetic layer for a fixed layer thickness is limited by the coercivity that can be written. Increasing V by increasing inter-granular exchange can also increase thermal stability. However, this approach could result in a reduction in the SMNR of the magnetic layer.
Thus, there is a need for new materials for the magnetic recording layer that provide increased grain segregation in the magnetic layer, leading to higher Ms and improved recording performance.